Method for transistor gate dielectric layer with uniform nitrogen concentration

ABSTRACT

The instant invention describes a method for forming a dielectric film with a uniform concentration of nitrogen. The films are formed by first incorporating nitrogen into a dielectric film using RPNO. The films are then annealed in N 2 O which redistributes the incorporated species to produce a uniform nitrogen concentration.

CROSS-REFERENCE TO RELATED PATENT/PATENT APPLICATIONS

[0001] The following co-assigned pending patent applications are herebyincorporated by reference: Patent No./Serial TI Case No. Filling DateNumber Inventors 09/291,844 04/14/99 TI-23502.1 Hattangady

FIELD OF THE INVENTION

[0002] The invention is generally related to the field of semiconductordevices and more specifically to method for forming a transistor gatedielectric layer with uniform nitrogen concentration.

BACKGROUND OF THE INVENTION

[0003] Remote Plasma Nitrided Oxides (RPNO) (or RPN oxynitrides) haverecently shown great promise as a gate dielectric layer in deepsubmicron CMOS. Its advantages include lower gate leakage by virtue of athicker dielectric film to achieve the same electrical oxide thickness,a lack of mobility degradation commonly associated with otheroxynitrides, excellent boron penetration resistance, and improved PMOSdrive current. RPNO films have been demonstrated to have reliabilitysuperior to that of a pure silicon oxide film when the overall filmthickness is less than about 23 angstroms. Thicker RPNO films havereliability only comparable to that of silicon oxide. This is believedto be due to the non-uniform nitrogen profile obtained in RPNO films. Amethod to form RPNO films is described in TI-23502.1, application Ser.No. 09/291,844, “Semiconductor device having interfacial nitrogen layersand method of formation”, and is hereby incorporated by reference. Thecurve shown in FIG. 1 represents a typical nitrogen profile that wouldbe obtained from a RPNO film using a measurement technique such as SIMSor Auger analysis. Region 100 represents the RPNO film and region 110the silicon substrate on which the RPNO film is formed. The nitrogenconcentration 115 is seen to peak at the surface of the RPNO film 100and decrease towards the silicon substrate 110. Current MOSFETtechnology requires that the reliability of the gate dielectric layerexceed that of pure silicon oxide. There is therefore a need for atransistor gate dielectric layer with reliability exceeding that of puresilicon oxide.

SUMMARY OF THE INVENTION

[0004] Accordingly, a need has arisen for a semiconductor device which agate dielectric layer with a uniform concentration of nitrogen. Inaccordance with the present invention, a transistor is described thatincludes a uniform nitrogen concentration in the gate dielectric layerand uses processing techniques that substantially eliminate or reducedisadvantages associated with prior devices and methods of formation.

[0005] According to one embodiment of the present invention, a silicondioxide film is exposed to a nitrogen containing plasma incorporatingnitrogen into the film. A thermal anneal is performed in a N₂O ambientto redistribute the incorporated nitrogen and produce a uniformdistribution of nitrogen in the film.

[0006] One advantage of the above described method is the forming of anasymmetric transistor without a degradation in transistor performance.This and other technical advantages of the instant invention will bereadily apparent to one skilled in the art from the following FIGUREs,description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] In the drawings:

[0008]FIG. 1 is a plot showing the nitrogen concentration profileobtained for a RPNO dielectric layer.

[0009]FIG. 2 is a cross-section diagram showing a typical MOS transistorwith a gate dielectric formed according to an embodiment of the instantinvention.

[0010] FIGS. 3(a)-3(d) are cross-section diagrams and plots showing anembodiment of the instant invention.

[0011] FIGS. 4(a)-4(c) show time of flight (TOF) SIMS profiles obtainedfrom dielectric layers fabricated according to an embodiment of theinstant invention.

[0012] Corresponding numerals and symbols in the different figures referto corresponding parts unless otherwise indicated.

DETAILED DESCRIPTION OF THE INVENTION

[0013] The invention will now be described in conjunction with the gatedielectric layer of a MOSFET transistor. It will be apparent to those ofordinary skill in the art that the benefits of the invention can beapplied to other semiconductor devices.

[0014] Shown in FIG. 2 is a MOS transistor with a gate dielectric layer50 formed according to an embodiment of the instant invention. Duringnormal transistor operation the gate dielectric layer 50 undergoesconstant stress from injected hot electrons or holes. Hot electrons orholes are those particles that have acquired enough energy whiletraversing the channel of the transistor to surmount the gate dielectriclayer/silicon substrate energy barrier and enter the gate dielectriclayer 50. The carriers that are injected into the gate dielectric layer50 can create defects in the dielectric layer 50 and at the dielectriclayer/silicon substrate interface 90 which can reduce the operatinglifetime of the transistor. A measure of how long the transistoroperates under certain conditions (i.e. its operating lifetime) isdetermined by the reliability of the gate dielectric layer 50. Thereliability of the gate dielectric layer 50 is therefore an importantproperty of the transistor. In an embodiment of the instant invention ithas been found that a gate dielectric layer which comprises siliconoxynitride with a uniform nitrogen concentration which is greater thatabout 6 atomic percent (6 atm. %) provides improved transistorreliability over existing gate dielectric schemes. This improvementoccurs for gate dielectric layers with a layer thickness less than about40 A. In the instant invention the concept of uniform nitrogenconcentration in a layer describes a less than 10% variation in nitrogenconcentration across the layer. thickness.

[0015] The transistor shown in FIG. 2 is fabricated using standardprocessing techniques. A gate dielectric layer 50 with a uniformnitrogen concentration greater than 6 atomic percent is formed on asilicon substrate 10 according to an embodiment of the instantinvention. A conductive gate layer 60 is formed on the gate dielectriclayer 50 and patterned to form a gate structure. Sidewall structures 70and formed adjacent to the patterned conductive layer 60 and the sourceand drain regions along with drain and source extensions 80 are formedin the substrate. Additional processing steps could be added to theabove described process if different transistor characteristics aredesired. A method for the formation of a gate dielectric layer withuniform nitrogen concentration according to an embodiment of the instantinvention will be described below.

[0016] A embodiment of the present invention illustrating the formationof a gate dielectric layer is illustrated in FIGS. 3(a)-3(d). Referringto FIG. 3(a), a semiconductor substrate 10 is provided that comprisessuitable materials such as silicon or gallium arsenide. For the case ofa silicon substrate, a silicon oxide layer 15 is formed on the surfaceof the silicon substrate 10. A number of silicon surface preparationtechniques such as cleaning and etching could be performed beforeforming the oxide layer 15. The oxide layer 15 will be on the order of40 angstroms or less in thickness.

[0017] Referring to FIG. 3(b), nitrogen in introduced into the oxidelayer 15 by subjecting the substrate 10 and oxide layer 15 to ahigh-density plasma of molecular nitrogen (N₂) or molecular nitrogen(N₂) carried along with some inert gas such as helium (H₂). Theresulting nitrogen containing oxide film (or RPNO film, or RPNoxynitride film, or oxynitride film) 20 has a nitrogen concentrationprofile which is highest at the surface of the film 20 and decreasestowards the substrate 10. The nitrogen profile obtained is shown in FIG.3(c). The high-density plasma can be generated with a number ofdifferent sources including but not limited to helicon,helical-resonator, electron-cyclotron resonance, and inductivelycoupled. For example, in the case of helicon, high-density, low pressureRF-generated plasma powered by a 13.56 MHz generator, the substrate 10and oxide layer 15 may be subjected to a 700-900 watt plasma at roomtemperature without substrate bias for a duration of about 30-80seconds. The molecular nitrogen flow should be on the order of 4millitorr. It should be understood that the process parameters describedpreviously are presented solely for the purpose of teaching theadvantages of the present invention and that other suitable processesfor including nitrogen for forming the film 20 may be used withoutdeparting from the intended scope of the present invention. The processdescribed is an extremely low pressure process to provide for thehighest ion density and the ion-flux available over the shortest time.Higher pressures may result in ion recombination which consequentlyreduces ion density. The process also uses a very low plasma potentialto reduce ion energies as much as possible. High ion energies can easilydamage the extremely thin oxide layer 15. Ion energies are thereforereduced by using no wafer or substrate bias or a wafer bias that is assmall as possible. A helicon wave based plasma generator usingmultipolar magnetic confinement is therefore well suited for the processof the present invention. Solely as an example, one suitable plasmasource is the MORI™ 2000 High Density Plasma Source manufactured byP.M.T. Those skilled in the art will recognize that other plasma systemsand configurations may also be used without departing from the scope ofthe present invention.

[0018] Referring to FIG. 3(c), the RPN oxynitride layer 20 formed byexposing an oxide film to a high-density N₂ plasma is annealed in N₂O attemperatures from 800-1100° C. This anneal is typically a rapid thermalanneal (RTA) for 10-60 seconds. In addition to RTA, furnace anneals inN₂O could also be used. The N₂O anneal will redistribute the nitrogenconcentration profile of the RPN oxynitride layer 20 shown in FIG. 3(c)to the uniform nitrogen concentration profile shown in FIG. 3(e). Theresulting oxynitride film 25 shown in FIG. 3(d), which is formed by theN₂O anneal of layer 20, has uniform nitrogen concentration and is thussuitable for use as the transistor gate dielectric layer 50 for the MOStransistor shown in FIG. 2. The redistribution of the nitrogenconcentration in the RPN oxynitride layer is believed to be due to thescavenging action of N₂O. During the annealing process, N₂O breaks upinto a number species including NO and O. It is believed that these NOand O species can react with the nitrogen in the RPN oxynitride layer 20effectively removing nitrogen from the layer 20. It is also believedthat the N₂O anneal in addition to removing nitrogen from the surface ofthe layer 20, will incorporate nitrogen at the interface of theoxynitride layer 20 and the substrate 10. The combined action of bothscavenging nitrogen from the surface of the RPN oxynitride layer 20 andincluding nitrogen at the interface of the oxynitride layer 20 and thesubstrate 10 during the N₂O annealing process results in the uniformnitrogen concentration shown in FIG. 3(e) for oxynitride layer 25.

[0019] Shown in the table below are the processing conditions used toform oxynitride layers whose nitrogen concentration profiles are shownin FIGS. 4(a)-4(c). Oxynitride layer RPN conditions N₂O annealing 1 800W/45 sec; N₂ + H₂ None (O₂ anneal at 1000° C. for 60 secs.) 2 800 W/45sec; N₂ + H₂ 900° C. for 20 secs. 3 800 W/60 sec; N₂ + H₂ 900° C. for 20secs.

[0020] The nitrogen concentration, oxygen concentration, and hydrogenconcentration depth profiles shown in FIGS. 4(a)-4(c) were all obtainedusing the time of flight (ToF) SIMS concentration profiling technique.Shown in FIG. 4(a) are the concentration depth profiles obtained for RPNoxynitride layer 1. The processing conditions used to form layer 1 aregiven in the table. An initial silicon oxide layer of about 23 A wasfirst formed. The silicon oxide layer was then exposed to a high-densitynitrogen plasma with a power level of 800 W for 45 seconds. This wasfollowed by a oxygen (O₂) anneal at 1000° C. for 60 seconds. Thenitrogen concentration profile 120 shown in FIG. 4(a) shows a peak atthe surface of the layer which decreases towards the substrate. Thenitrogen concentration varies from about 12 atomic percent of nitrogenat the surface to a value close to zero at the substrate surface. Thelarge nitrogen tail observed below the substrate surface is an artifactof the measurement technique. Shown in FIG. 4(b) are the concentrationprofiles obtained from an oxynitride layer formed by exposing a 23 Asilicon oxide to a N₂ plasma at 800 W for 45 seconds followed by a RTAN₂O anneal at 900° C. for 20 seconds. The scavenging action of the N₂Oanneal has reduced the surface concentration of nitrogen from about 12atomic to about 8 atomic percent resulting in an oxynitride film with auniform nitrogen concentration of about 8 atomic percent. Shown in FIG.4(c) are the concentration profiles obtained from an oxynitride layerformed by exposing a 23 A silicon oxide to a N₂ plasma at 800 W for 60seconds followed by a RTA N₂O anneal at 900° C. for 20 seconds.Following the N₂O anneal an oxynitride film with a uniform nitrogenconcentration of about 10 atomic percent is obtained. The resultinguniform N₂ concentration level in film is therefore determined both bythe initial RPN process and the N₂O anneal.

[0021] The instant invention teaches a method for forming an oxynitridelayer with uniform nitrogen concentration. These oxynitride layers aresuitable for use as gate dielectric layers in MOS transistors. In anembodiment of the instant invention, the reliability of the MOStransistor (i.e. its immunity to hot carrier degradation) is improvedover that of pure silicon oxide if the uniform nitrogen concentration isabove 6 atomic percent for a gate dielectric layer thickness less than40 angstroms. An additional advantage of the instant invention is thatthe layers formed have no measurable hydrogen. This lack of measurablehydrogen is shown in FIGS. 4(a)-4(c) for oxynitride layers formedaccording to an embodiment of the instant invention.

[0022] While this invention has been described with reference toillustrative embodiments, this description is not intended to beconstrued in a limiting sense. Various modifications and combinations ofthe illustrative embodiments, as well as other embodiments of theinvention, will be apparent to persons skilled in the art upon referenceto the description. It is therefore intended that the appended claimsencompass any such modifications or embodiments.

We claim:
 1. A method for forming a MOS transistor gate dielectric layercomprising: providing a semiconductor substrate; forming an oxide layeron the semiconductor substrate; exposing the oxide layer to ahigh-density nitrogen plasma to incorporate nitrogen into the oxidelayer thereby converting the oxide layer to an oxynitride layer; andannealing said oxynitride layer in N₂O to form an oxynitride layer witha uniform nitrogen concentration profile.
 2. The method of claim 1wherein the exposing the oxide layer to a high-density nitrogen plasmacomprises a plasma power level of 700-900 watts.
 3. The method of claim1 wherein annealing the oxynitride layer in N₂O comprises rapid thermalannealing at a temperature of 800° C.-1100° C. for 10-60 seconds.
 4. Amethod of forming a MOS transistor comprising: providing a semiconductorsubstrate; forming a gate dielectric layer on the semiconductorsubstrate wherein the gate dielectric layer has a uniform nitrogenconcentration; forming a conductive layer on said gate dielectric layer,forming sidewall structures adjacent to said conductive layer; andforming source and drain regions in the semiconductor substrate adjacentto said sidewall structures.
 5. The method of claim 4 wherein saidforming a gate dielectric layer with a uniform nitrogen concentrationcomprises: forming an oxide layer on the semiconductor substrate;exposing the oxide layer to a high-density nitrogen plasma toincorporate nitrogen into the oxide layer thereby converting the oxidelayer to an oxynitride layer; and annealing said oxynitride layer in N₂Oto form an oxynitride layer with a uniform nitrogen concentrationprofile.
 6. The method of claim 5 wherein the exposing the oxide layerto a high-density nitrogen plasma comprises a plasma power level of700-900 watts.
 7. The method of claim 5 wherein annealing the oxynitridelayer in N₂O comprises rapid thermal annealing at a temperature of 800°C.-1100° C. for 10-60 seconds.
 8. The method of claim 4 wherein saiduniform nitrogen concentration is greater than 6 atomic percent.
 9. Themethod of claim 4 wherein gate dielectric layer is less than 40angstroms thick.
 10. The method of claim 4 wherein said uniform nitrogenconcentration describes a nitrogen concentration with less than 10%variation across the gate dielectric layer.
 11. A MOS transistor,comprising: providing a silicon substrate; a gate dielectric layer onthe silicon substrate wherein the gate dielectric layer is less than 40angstroms thick and wherein the gate dielectric layer has a uniformnitrogen concentration; a conductive layer on the gate dielectric layer;sidewall structures adjacent to said conductive layer; and source anddrain regions in the silicon substrate adjacent to the sidewallstructures.
 12. The MOS transistor of claim 10 wherein the uniformnitrogen concentration is greater than 6 atomic percent.
 13. The MOStransistor of claim 12 wherein the uniform nitrogen concentration has avariation of less than 10% across the gate dielectric layer.